The strong light-matter interaction and the valley selective optical selection rules make monolayer (ML) MoS 2 an exciting 2D material for fundamental physics and optoelectronics applications. But, so far, optical transition linewidths even at low temperature are typically as large as a few tens of meV and contain homogeneous and inhomogeneous contributions. This prevented in-depth studies, in contrast to the bettercharacterized ML materials MoSe 2 and WSe 2 . In this work, we show that encapsulation of ML MoS 2 in hexagonal boron nitride can efficiently suppress the inhomogeneous contribution to the exciton linewidth, as we measure in photoluminescence and reflectivity a FWHM down to 2 meV at T ¼ 4 K. Narrow optical transition linewidths are also observed in encapsulated WS 2 , WSe 2 , and MoSe 2 MLs. This indicates that surface protection and substrate flatness are key ingredients for obtaining stable, high-quality samples. Among the new possibilities offered by the well-defined optical transitions, we measure the homogeneous broadening induced by the interaction with phonons in temperature-dependent experiments. We uncover new information on spin and valley physics and present the rotation of valley coherence in applied magnetic fields perpendicular to the ML.

We have investigated the exciton dynamics in transition metal